Various vehicle door driving devices have been conventionally proposed. In the vehicle door driving device described in Patent Document 1, a guide rail is fixed to a body of a vehicle. The door of the vehicle is supported to be movable with respect to the body by way of the guide rail. The vehicle door driving device includes a driving mechanism, which is fixed to the door, and a cable, which is selectively reeled in and out by the driving mechanism. The ends of the cable are coupled to a front end and a rear end, respectively, of the guide rail at the body of the vehicle. Thus, the door opens and closes when the cable is selectively reeled in and out by the driving mechanism.
Patent Document 2 describes a driving mechanism that can be applied to the vehicle door driving device described above. The driving mechanism described in the document adopts a planetary gear train for a speed reducing mechanism. The planetary gear train includes a sun gear serving as a driving shaft, a ring gear serving as a driven shaft, and a carrier serving as a fixed shaft. The sun gear is directly coupled to a rotary shaft of a motor. The ring gear is coupled to a drum to rotate with the drum. The rotation of the motor is decelerated by the planetary gear train, and then transmitted to the drum. The speed reducing mechanism is configured such that each gear is arranged coaxially with the rotary shaft of the motor. A projected area in an axial direction of the driving mechanism is thus reduced.
Patent Document 3 describes another driving mechanism that can be applied to the vehicle door driving device described above. This driving mechanism described in the document adopts a worm gear, which has high reduction ratio, for the speed reducing mechanism. The driving mechanism described in the document includes an electromagnetic clutch that connects/shields the rotation transmitted from the motor to the drum through the speed reducing mechanism. The outer diameter of the drum is set based on a reeling speed, necessary torque, and the like with respect to the output characteristics of the motor. The outer diameter of the electromagnetic clutch is set to be smaller than the outer diameter of the drum. In this case, the dimension in the axial direction of the electromagnetic clutch becomes larger by reducing the outer diameter of the electromagnetic clutch. However, the enlargement of the electromagnetic clutch is suppressed by accommodating the electromagnetic clutch in the drum and effectively utilizing the space in the drum.
In the driving mechanism of Patent Document 2, the dimension in the axial direction of the driving mechanism is large because the motor and the speed reducing mechanism are stacked in the axial direction thereof. Thus, it is difficult to mount the driving mechanism on the door of the vehicle where space is restricted.
In the driving mechanism of Patent Document 3, the worm gear that decelerates at an axial portion of the motor is adopted. The space around the rotary shaft of the motor thus cannot be effectively utilized. In this case, the dimension in the axial direction of the driving mechanism becomes large because an output shaft (worm wheel) of the worm gear, the drum, and the electromagnetic clutch are stacked in the axial direction thereof. A large motor needs to be used to obtain the desired torque because the worm gear, which has low transmission efficiency, is adopted for the speed reducing mechanism.